The present invention relates to the biomedical arts for introducing electrical signals on nerve trunks. The present invention finds particular application in introducing a string of artifically generated antidromic pulses on the nerve trunk for collision blocking orthodromic pulses moving in the opposite direction along the nerve trunk and will be described with particular reference thereto. It is to be appreciated, however, that the invention may have broader applications and may apply electrical signals on nerve trunks for other purposes.
Heretofore, various techniques have been used to block nerve pulses passing along a nerve trunk. A common blocking technique was the application of DC currents on the nerve trunk. However, it has been found that the application of DC currents can be expected to cause nerve damage.
To eliminate the DC current induced nerve damage, others have suggested using an oscillating current such that the induced electrical current flowed alternately in both directions along the nerve trunk. It has been found that the application of high frequency stimulation blocks the passage of nerve signals therethrough. However, it appears that high frequency stimulation may, in effect, be overdriving neuromuscular junctions and depleting the neurotransmitter at the terminal end. That is, rather than blocking the passage of nerve stimuli on the nerve fiber or axon, the high frequency stimulation techniques may be overworking the nerve terminal to the point of exhaustion causing a failure of proper functioning.
Yet another blocking technique utilized a three electrode cuff which included a dielectric sleeve having a passage through which the nerve trunk passes. Three annular electrodes were arranged within the sleeve. A cathode was positioned near the center of the passage and a pair of anodes were positioned to either side. A signal generator was connected with the electrodes to apply an electrical pulse train that induced antidromic pulses on the nerve trunk. Each pulse of the pulse train included a rapid rise to a preselected amplitude, a 100 to 3000 microsecond plateau, and an exponential decay back to zero. This pulse train induced artifically generated antidromic pulses on the nerve trunk which traveled unidirectionally in the opposite direction to the normal pulse flow. The artificially generated antidromic pulses collided with and blocked further propagation of natural orthodromic pulses moving in the other direction on the nerve trunk. However, the application of a series of pulses of common polarity, again has been found to cause damage to neural tissues.
To eliminate this nerve damage, others have suggested applying a low amplitude, relatively long duration rectangular wave pulse of opposite polarity between each pulse of the above-described pulse train. The opposite polarity of the rectangular wave pulse balanced the net charge flow caused by the primary pulse. However, it has been found that at an upper limiting frequency, the sudden polarity change still tends to depolarize the nerve cell and cause transmission in the wrong direction. This tendency to generate artificial orthodromic pulses, of course, was undesirable. For example, if the antidromic blocking pulses were utilized to block stray excitation pulses moving toward a paralyzed patient's spastically contracted sphincter muscle over which control had been lost, the stray orthodromic pulses would cause undesired activation of the muscles of micturition.
The present invention contemplates a new and improved method and apparatus for artifically generating antidromic pulses.